1. Field of the Invention
The present invention relates to a method for detecting radial tilt of an optical recording medium in an optical head device for performing recording/reproducing on an optical recording medium, an optical head device, and an optical information recording/reproducing device. More specifically, it relates to a method for detecting radial tilt of an optical recording medium in an optical head device, which is capable of detecting radial tilt of a read-only type optical recording medium, an optical head device, and an optical information recording/reproducing device.
2. Description of the Related Art
The recording density in an optical information recording/reproducing device is inversely proportional to the square of the diameter of focused spot which is formed on an optical recording medium by an optical head device. Thus, the smaller the diameter of the focused spot is, the larger the recording density becomes. The diameter of the focused spot is inversely proportional to the numerical aperture of an objective lens in the optical head device. Thus, the higher the numerical aperture of the objective lens is, the smaller the diameter of the focused spot becomes.
On the other hand, when the optical recording medium tilts against the objective lens in the radial direction, the shape of the focused spot becomes disturbed due to coma aberration caused by a substrate of the optical recording medium so that the recording/reproducing characteristics become deteriorated. The coma aberration is inversely proportional to the cube of the numerical aperture of the objective lens. Thus, the higher the numerical aperture of the objective lens is, the narrower the margin of the tilt (radial tilt) of the optical recording medium in the radial direction for the recording/reproducing characteristics becomes.
Therefore, in the optical head device with an increased numerical aperture of the objective lens for increasing the recording density, it is necessary to detect and correct the radial tilt of the optical recording medium in order not to deteriorate the recording/reproducing characteristics.
FIG. 9(A) shows the structure of a conventional optical head device capable of detecting radial tilt of a read-only type optical recording medium.
This optical head device is disclosed in Japanese Patent Application Laid-open No. 2001-110074. The light emitted from a semiconductor laser 1 is collimated by a collimator lens 2, and a part of the emitted light transmits through a beam splitter 20 to be focused on a disk 7 by an objective lens 6.
The reflected light from the disk 7 inversely transmits through the objective lens 6, and a part of which is reflected by the beam splitter 20 and transmits through a lens 9 to be received by a photodetector 10b. 
FIG. 9(B) shows both a pattern of a light receiving section of the photodetector 10b and the structure of a circuit for detecting radial tilt of a disk 7. The light receiving section of the photodetector 10b is divided into eight light receiving sections A1, A2, B1, B2, C1, C2, D1, and D2 by three division lines parallel to tangential direction of the disk 7 and a division line parallel to the radial direction. The phase difference of the signals outputted from the light receiving sections A2, B2 and the phase difference of the signals outputted from the light receiving sections C2, D2 are obtained by phase comparators 21a and 21b, respectively. The obtained values are added using an adder 22 to detect radial tilt of the disk 7.
In the conventional optical head device, the reflected light from the disk 7 is received by the eight light receiving sections. Therefore, compared to an ordinary optical head device in which reflected light from an optical recording medium is received by four light receiving sections and which does not detect radial tilt, there requires twice the number of the current-voltage conversion circuits for converting the current outputted from the light receiving sections to voltage. When the number of the current-voltage conversion circuits increases, noise caused by the circuit increases thereby reducing the signal to noise ratio of an RF signal.